1. Field of the Invention
The present invention relates to an apparatus for generating virtual element images, particularly to a virtual input element image projection apparatus.
2. Related Art
Being slim and light and having a low noise level are the prevailing trends for the development of portable electronic products. The commonly used keyboard input element, due to visual constraint and ergonomic dimensional consideration, has little room for further shrinking. Moreover, user's habits mostly still incline to using a larger keyboard input element. Although there are many alternative input elements, such as a hand written panel, voice, light pen, and the like. To the great user population who are used to keyboard input, the habit of using keyboard input is unlikely to be substituted by other methods.
Hence the virtual keyboard was developed and introduced. For users who are accustomed to using the keyboard, it does not generate noise, takes little space, and is portable. It generally adopts a digital micro mirror display technique to project the keyboard image. U.S. Pat. No. 6,266,048 discloses such a technique. It displays keyboard and screen patterns on a flat surface by projection, and uses an electronic device to emit two laser beams, and has two optical sensors on the bottom to detect light reflection angles while user's fingers do input operation. The angles detected by the two optical sensors are used to determine finger-input positions on the keyboard. It requires complicated signals, controlling laser light, to do a two-dimensional scan rapidly. To adopt that technique for inputting devices on consumer electronic products, is too costly and cannot meet market requirements.
There is another technique that employs diffractive optical elements (DOE) in the virtual keyboard image projection apparatus. U.S. Pat. No. 6,614,422 discloses such a technique. It has a diffractive projection system, which contains at least one diffractive optical element to project virtual keyboard images. According to the diffraction principle, the emission angle of the virtual keyboard image generated by the diffractive optical element is determined by forming line width of the diffractive optical element. The smaller the line width, the bigger the diverging angle of the keyboard image becomes. Thus it can generate a large angle diverging keyboard image at a relatively short distance. However, due to technical limitation of the semiconductor manufacturing process, the smaller line width requires a more complex fabrication process. Moreover, the diffractive optical elements mostly are multi-stage structured, and require a higher precision in the fabrication process. With the diffractive optical elements having a narrower line width, the diffraction efficiency decreases. Due to the foregoing reasons, the virtual keyboard images generated by the projection apparatus that employ diffractive optical elements do not have a large enough diverging angle.